Primary photochemistry of iron-depleted and zinc-reconstituted reaction centers from Rhodopseudomonas sphaeroides

The primary photochemistry of Fe-depleted and Zn-reconstituted reaction centers from Rhodopseudomonas sphaeroides R-26.1 was studied by transient absorption spectroscopy and compared with native, Fe(2+)-containing reaction centers. Excitation of metal-free reaction centers with 30-ps flashes produced the initial charge-separated state P(+)I(-) (P(+)BPh(-), where P is the primary donor and BPh is bacteriopheophytin) with a yield and visible/near-infrared absorption difference spectrum indistinguishable from that observed in native reaction centers. However, the lifetime of P(+)I(-) was found to increase approximately 20-fold to 4.2 +/- 0.3 ns (compared to 205 ps in native reaction centers), and the yield of formation of the subsequent state P(+)Q(A) (-) (Q(A) is the primary quinone acceptor) was reduced to 47 +/- 5% (compared to essentially 100% in native reaction centers). The remaining 53% of the metal-free reaction centers were found to undergo charge recombination during the P(+)I(-) lifetime to yield both the ground state (28 +/- 5%) and the triplet state P(R) (25 +/- 5%). Reconstitution of Fe-depleted reaction centers with Zn(2+) restored the "native" photochemistry. Possible mechanisms responsible for the reduced decay rate of P(+)I(-) in metal-free reaction centers are discussed.